1. Field of the Invention
This invention relates to the internals of a nuclear reactor and more specifically to a strainer which prevents debris from lodging between the secondary lower core support and the hemispherical lower head on the pressure vessel.
2. Prior Art
A pressurized water reactor includes an upright cylindrical pressure vessel with a hemispherical lower head section and a removable hemispherical head bolted to the upper end. A cylindrical core barrel is suspended inside the pressure vessel from a flange extending around the inside of the upper end of the pressure vessel. The core of fissile material is supported inside this core barrel. An upper core support, commonly referred to as the top hat, is clamped down on top of the flange from which the core barrel is suspended by the removable hemispherical upper head. In order to accommodate for tolerances, an annular spring is placed between the flanges on the core barrel and the upper core support.
Reactor coolant is introduced into the pressure vessel near the top of the core barrel, flows downward through the annular space between the core barrel and the inner wall of the pressure vessel known as the downcomer, reverses direction inside the hemispherical lower head section, and flows upward through passages in the bottom of the core barrel and through the reactor core mounted inside the core barrel before being discharged through outlet nozzles. Heat energy generated by the fission reactions in the core is absorbed by the reactor coolant and is utilized to generate steam for use by a turbine-generator in producing electricity.
As a safety precaution, energy absorbers are mounted under the suspended core barrel so that in the very unlikely event that there should be a complete failure of the core barrel suspension system, the impact of the entire core barrel assembly falling on the lower hemispherical head section is lessened to preserve pressure vessel integrity. In such an event, actual contact with the lower hemispherical head would be made by a horizontal secondary core support plate underneath the energy absorbers. In order to keep the kenetic forces in such an accident to a minimum, the gap between the secondary core support plate and the lower hemispherical head section of the pressure vessel is very small. However, due to the differences in the coefficients of thermal expansion of the stainless steel internals and the lower alloy pressure vessel, this gap varies in size over the temperture range to which these components are exposed. Typically, this gap can narrow from 1.06 inches cold to 0.5 inches when the reactor is operating at full power.
Periodically, the reactor is shutdown for refueling. During this sequence, the hemispherical upper head is removed along with the upper core support and the components it supports so that fuel assemblies can be replaced and rearranged. The possibility exists during the refueling procedure for debris to fall down into the lower portion of the vessel. Such debris can include small parts, such as nuts and bolts. Debris can also be introduced into the pressure vessel by circulation of reactor coolant following failure of the hardware or maintenance on other parts of the nuclear steam supply system. For instance, during retubing of the steam generators, small pieces of tubing and pieces of weld material can be left behind despite attempts to clean them out.
If such debris should lodge between the secondary lower core support and the lower hemispherical head section of the pressure vessel when the reactor coolant is cold, it could cause the core barrel to unseat from its support flange as the components heat up, due to the difference in the coefficients of thermal expansion of the internals and the pressure vessel. This in turn, could subject the internals to undesired vibration induced by turbulent reactor coolant flow.
As taught by U.S. Pat. No. 4,096,032, it is known to insert filters in the bottom of the core barrel of a pressurized water reactor temporarily during cold hydrostatic and hot functional testing to collect debris from construction of the nuclear steam supply system. However, this is done before the fuel assemblies are installed and it collects the debris in the lower hemispherical head section of the pressure vessel from which it must be removed prior to operation of the reactor or it will cause the very problem which the present invention seeks to avoid.
It is the primary object of the present invention to provide apparatus which prevents debris from lodging in the gap between the internals and the lower hemispherical head section of the reactor pressure vessel without restricting the flow of reactor coolant, and which does so as the gap varies in size over the full range of temperatures to which the reactor is subjected.